opcodes: aarch64: flags to denote subclasses of arithmetic insns

[Changes in V4]
- Specify subclasses only for those iclasses relevent to SCFI:
  addsub_imm, and addsub_ext
[End of changes in V4]

[No changes in V3]
[New in V2]

Use the three new subclass flags: F_ARITH_ADD, F_ARITH_SUB,
F_ARITH_MOV, to indicate add, sub and mov ops respectively.

opcodes/
    * aarch64-tbl.h: Use the new F_ARITH_* flags.